1. Field of the Invention
The present invention relates to a wide-angle lens system which is used in a single lens reflex (SLR) camera.
2. Description of the Prior Art
In a SLR camera, since a quick-return mirror is provided between the photographing lens system and the image plane, a back focal distance of a predetermined length is required for positioning the quick-return mirror. Accordingly, in a wide-angle lens system, a retrofocus lens system which has a back focal distance longer than the focal length is generally employed. Such a retrofocus lens system includes a front lens group having a negative refractive power (hereinafter, a negative front lens group), a diaphragm, and a rear lens group having a positive refractive power (hereinafter a positive rear lens group), in this order from the object. Since the distribution of refractive power is asymmetric with respect to the diaphragm, various aberrations, such as spherical aberration, coma, distortion, and astigmatism, tend to be larger. In order to correct such aberrations, it is necessary to increase the number of lens elements. Furthermore, in a lens system in which all the lens groups are advanced upon focusing, there is a need to increase the diameter of the front lens group to collect peripheral illumination. The front lens group of such a large diameter is remarkably seen in a fast super-wide-angle lens system having an f-number of 2.8 and the half angle-of-view of more than 50xc2x0. However, a lens system, in which the diameter of the front lens group is large and the number of lens elements is large, is disadvantageous for an automatic focusing (AF) operation, mainly because the weight of the lens groups to be moved becomes heavy. It is therefore preferable to employ an internal focusing lens system.
However, even in the case of an internal focusing lens system, if a super-wide-angle lens system is employed, the number of lens elements in the negative front lens group have to be increased in order to correct field curvature and distortion which are influenced by the angle-of-view, so that the thickness of the negative front lens group is increased. As a result, the diameter of the front lens group increases, and it becomes difficult for a user to handle the lens system.
The present invention provides a wide-angle lens system of inner-focusing and retrofocus type having three lens groups, in which the half angle-of-view of at least 45xc2x0 (preferably, more than 50xc2x0), and an f-number of approximately between 2.8 and 4.0 are achieved.
As an aspect of the present invention, there is provided a wide-angle lens system including a negative first lens group, a positive second lens group, a diaphragm, and a positive third lens group, in this order from the object. Upon focusing, the positive second lens group moves along the optical axis direction. The negative first lens group includes at least one negative lens element having an aspherical concave surface facing toward the image, and the aspherical surface satisfies the following condition:
1.0 less than X1/f less than 3.0xe2x80x83xe2x80x83(1) 
wherein
X1 designates the distance between the aspherical surface on the optical axis to the diaphragm, and
f designates the focal length of the entire wide-angle lens system at an infinite photographing distance.
The aspherical surface formed on the concave surface of the negative lens element is preferably formed so that (i) the radius of curvature of the aspherical surface increases as an increase of a height from the optical axis, and (ii) satisfies the following condition:
xe2x88x920.20 less than (xcex94X/f)*(Nxe2x88x921) less than xe2x88x920.01xe2x80x83xe2x80x83(2) 
wherein
xcex94X designates the amount of asphericity at a height Y (Y=0.33*(X1)), and
N designates the refractive index of the lens material of the lens element on which the aspherical lens surface is formed.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-229807 (filed on Jul. 30, 2001) which is expressly incorporated herein in its entirety.